1. Field of the Invention
The present invention relates in general to collection of ground-water samples during vertical profiling of ground-water quality in the subsurface and, more particularly, to a removable plug and collar system for use with a hollow stem screened auger.
2. Description of the Related Art
Hollow stem drilling augers are commonly used to bore holes in the earth to collect water and soil samples and to construct monitoring wells. A hollow stem auger has a cylindrical shape and an outer wall that is typically solid, but can be laser slotted or screened. An example of a screened auger 10 having slots 13 is shown in FIG. 1A. An example of a solid-wall auger 11 is shown in FIG. 1B. Hollow stem augers 10, 11 have a hollow center or passageway for receiving sampling tools. A cutting head or drilling bit 14 is attached to the bottom of the hollow stem auger 10, 11 to cut through soil. A continuous, helical flighting 12 is attached to the outer wall to lift the cut soil out of the bore hole. Hollow stem augers typically have an outside diameter of 6.25 inches (including the flighting) and a 3.25-inch inner diameter.
A series of hollow stem augers are connected end to end as the drilling process proceeds to form an auger column. FIG. 2 shows a typical hollow stem auger column 20. An uppermost auger 28 of the auger column 20 attaches to a drill rig (not shown) to rotate the auger column 20 into the soil. The drilling bit 14 is attached to the lowermost or leading auger. The auger column 20 provides a continuous bore from the surface level to the desired sample or drilling level. Boring and sampling tools are passed from the surface level through the hollow center of the auger column 20 to the desired depth. A single auger 10, 11 is typically 5 or 10 feet in length, enabling sampling of soil or water in aquifers at varying levels below the earth's surface.
When water is to be sampled, the screened auger 10 is used as the leading auger for the intake of ground fluids. The augers above the screened auger 10 are solid-wall augers 11. The augers 10, 11 are rotated and successively interconnected until the desired sampling level is reached. A sampler or submersible pump is lowered down the hollow stem to the screened auger 10 to collect a sample. To collect another sample at a different depth, the sampler or pump is removed, additional augers 11 are attached to the top of the auger column 20, and the augers 10, 11 are rotated down to the next desired level, where the sampling process is repeated. The sampler or pump must be retrieved prior to the connection of each additional auger 11 and then lowered once again after drilling to the next desired depth.
While drilling, formation materials can enter the hollow stem as the auger column 20 is advanced. Formation materials can be prevented from entering the hollow stem by inserting a center plug (not shown) at the bottom of the screened auger 10, which is knocked out and left in the ground prior to well installation or soil sampling. Formation materials can also be blocked by using a center rod assembly with an attached plug system (not shown) that is retrieved prior to well installation.
FIG. 3 illustrates how two adjacent augers in the auger column 20, such as the screened auger 10 and one of the solid-wall augers 11, are connected together. Opposite ends of each auger terminate in a socket end (female) connector 30 with a bolt hole 36 and a plug end (male) connector 32 with a bolt hole 38. As shown in FIG. 3, the socket end connector 30 of one of the solid-wall augers 11 receives the plug end connector 32 of the screened auger 10 until the socket end connector 30 contacts an auger stop 39. The joint between any two interconnected auger sections is secured using an auger bolt 34 to prevent the auger sections from slipping apart.
Collection of ground-water samples using conventional screened augers and submersible pumps is cumbersome and time consuming, and the reliability of the samples is questionable. At each sample depth, after the advancement of the screened auger into virgin aquifer, a pump must be lowered into the augers and placed at the depth of the screened auger. Because the screened auger, which is the first auger at the bottom of the auger column, is not isolated from the auger column above it, the pump is not isolated and water in the column above the screened auger can flow into the pump and jeopardize the reliability of the sample.
To provide a more reliable sample, an inflatable packer can be placed above the pump to isolate the water to be sampled from accumulated water in the hollow stem above the packer. The pump is turned on, water is purged from the isolated zone, and then a sample of the ground water is collected while the selected zone refills. However, with conventional auger drilling systems, it is necessary to lower the pump, inflate the packer, collect the sample, deflate the packer, and then remove the pump with the addition of each auger to further advance the bottom screened auger and collect a sample. For each new sample collected at a deeper depth, the process must be repeated. These operations are time-consuming and disturb the water column within the augers, which compromises the reliability of the sample.
Thus, there is a need for a system and method for reliable, efficient, and continuous collection of samples using a screened auger that prevents water above the screened auger from flowing into the pump within the screened auger, while allowing the installation of a monitoring well after vertical profiling, if desired.